1. Field of the invention
The present invention relates to a liquid crystal composition for an active matrix LCD and a liquid crystal display device which is obtained by using the composition.
2. Description of the Prior Art
By reason that, compared with CRT (a display of a Braun tube type ), it is possible to provide a liquid crystal display device (LCD) having properties of lower consumption of electric power, a small size and light weight, many kinds of LCD, such as a twist nematic (TN) mode, a super twist nematic (STN) mode and a thin film transistor (TFT) mode, have been utilized. In particular, an active matrix LCD (AM-LCD) such as a thin film transistor (TFT) and the like is noticed as an important flat display which is a colored and highly precision apparatus.
The AM-LCD is required to have the following special properties:
1) a high voltage holding ratio (V.H.R.) for maintaining high contrast of LCD, PA1 2) a wide operating range of temperatures showing a nematic liquid crystal phase, PA1 3) proper optical anisotropy (.DELTA.n) in proportion to cell thickness, and PA1 4) proper threshold voltage according to a driving circuit.
In the AM-LCD mode, a TN display mode having a structure twisting the alignment direction of liquid crystal molecules toward 90.degree. between a pair of upper and lower electrode substrate plates is adopted. This TN display mode needs to prevent coloring by interference of liquid crystal cells when no voltage is applied, and it needs to establish a certain value (for example, .DELTA.n.times.d=.about.0.5 .mu.m) of the product (.DELTA.n.times.d) of the optical anisotropy (.DELTA.n) and the cell thickness (d) .mu.m. The .DELTA.n of liquid crystal compositions for TFT utilized under such limitations have .DELTA.n of about 0.07-0.15, particularly 0.08-0.13 in the 1st. Min. type. When liquid crystal materials having high .DELTA.n are used, the value of d can be lowered. If the value of d is lowered, since the response time (abbreviated as .tau., hereinafter according to circumstances) is proportioned to the viscosity of liquid crystal materials (abbreviated as .eta., hereinafter according to circumstances) and to a square of the cell thickness d, the .tau. becomes remarkably lowered. The liquid crystal composition for a liquid crystal device having a proper high .DELTA.n is very useful.
In recent years, personal computers and the like which are characterized by a compact, light weight and of the portable note book type have been developed, so that the LCD becomes widely used. Characteristics of the LCD handy to carry are restricted by the driving power resource. Since the electric power consumption must be lowered for long-term use, liquid crystal compositions having low threshold voltage are required. Further, to obtain light and low-priced driving power resources, liquid crystals having low threshold voltage are required. The threshold voltage of liquid crystals for a 5 V driving equipment is practically about 2.0 V, and that of a 4 V driving equipment is about 1.6 V. Liquid crystal compositions having a threshold voltage of 1.5 V or less have recently become necessary.
In addition, following the small-sized LCD, development of equipment for use outdoors has studied. For use outdoors, a nematic phase over the range of outdoor ambient temperature is required. From this point of view, liquid crystal compositions of TFT for practical use have mainly a nematic-isotropic phase transition temperature (clearing point: T.sub.NI) of 60.degree. C. or more and a smectic-nematic phase transition temperature (T.sub.SN) of -20.degree. C. or less.
Following such backgrounds, a new compound and a liquid crystal composition using the compound are disclosed in Japanese Patent Publication H-5-500679-A. Though the composition has large .DELTA.n, it has too high a threshold voltage to use for low voltage driving. In the such publication, a composition for 2nd. Min. or PDLC is also disclosed. However, 1st. Min. is used in the field of AM-LCD. Accordingly, the composition disclosed in the publication H-5-500679-A has a disadvantage that the .DELTA.n is beyond the preferable range.
As described above, though the liquid crystal compositions are being studied by many objectives, new improvement is usually required.